The present invention relates generally to the processing of thermoplastic polymers and, more particularly, to an improved filtration and diverter valve arrangement for use in such systems for directing the flow of viscous molten thermoplastic polymers under relatively high pressures.
In many manufacturing operations involving the formation of products from molten thermoplastic material, e.g., plastic extrusion, injection molding, and blow molding operations, it is desirable, if not essential, that the operation be carried out on a continuous flow basis. Additionally, it is common practice in such operations to filter impurities from the thermoplastic material by passage through a suitable filtration unit. Necessarily, such filtration units require periodic cleaning and, accordingly, to accommodate such cleaning and also permit the manufacturing operation to proceed on a continuous basis, dual filtration units are typically utilized with a diverter valve situated in the flow line to selectively direct the fluid material through one of the filters while the other is inactive for periodic cleaning and maintenance and otherwise to serve as a back-up to the operating filter.
Molten thermoplastic polymers often have temperatures in excess of 400.degree. F. and even as high as 600.degree. F. or more and often flow at pressures in excess of 1000 PSI up to 5000 PSI. Importantly, many thermoplastic polymers can be quite sensitive to temperature, such that the residence time of the polymers flowing through the diverter valve and the operative filter must be closely controlled to avoid overheating and possible degradation of the polymer. Thus, such valves must be configured such that the fluid flow characteristics do not differ between the alternate operating dispositions of the valve and, especially, the flow passageways through the valve must be configured to avoid any stagnation in the fluid flow which could cause certain thermoplastic materials to undergo changes in their physical or chemical character. Similarly, it is critical to control operation of the diverter valve and the filters during changeovers from one filter to the other in order to ensure continuous polymer output flow without significantly altering the temperature and characteristics of the polymer.
Thus, the process of changing over a polymer process line from one filter to another by means of the diverter valve is normally carried out gradually by initially diverting only a portion of the polymer flow, e.g., approximately 10%, from the filter in active use which needs cleaning or maintenance while correspondingly increasing the polymer pumping speed to maintain the same volumetric output downstream from such filter. In this manner, the clean, idle filter can be filled with polymer in preparation for the ultimate changeover completely to the idle filter. The filters typically include a vent valve which may be located external to the lid of the filter vessel for bleeding the air and initial flow of polymer incoming into an idle filter preparatory to placement of the filter into operation. In order to maintain heat within the filter and avoid loss of polymer temperature, the external vent valve must be jacketed.
Once the changeover has been fully completed, the filter taken out of service can be disassembled for cleaning and/or maintenance. Such filters are typically contained within an enclosed vessel which itself is supported within a surrounding sleeve or jacket so that heated oil can be circulated about the filter vessel during operation in order to maintain the desired polymer temperature level. As will be understood, the piping from the diverter valve must therefore pass through the outer jacket or sleeve into sealed relation with the filter vessel therewithin in order to prevent leakage of the polymer. For this purpose, the inlet port on the filter vessel typically carries an annular seal which presses against the end of the piping extending into the sleeve or jacket when the filter vessel is installed properly within the jacket. Since such seals are quite fragile, considerable care must be taken when reinstalling a cleaned or repaired filter vessel into the sleeve in order to prevent damage to the seal from contact with the piping.
In order to press the annular seal of the filter vessel into sealed relation with the piping from the diverter valve, the sleeve or jacket surrounding the filter vessel typically includes two bolt assemblies, each including a bolt housing having an opening situated on the opposite side of the filter vessel from the inlet port for receiving the bolt. Once the filter vessel is properly installed into the sleeve or jacket, the bolt, operating through the opening, is adapted to press directly against the filter vessel in the direction of the diverter valve to push the annular seal of the inlet port into sealing engagement with the piping. When the filter is idle, the bolts are loosened and retracted and the out of service filter vessel can be removed and disassembled for cleaning and/or maintenance.
Ideally, the piping extending from the diverter valve to the two filters should be as short in length as possible, which facilitates cleaning. Two examples of conventional diverter valves used in polymer processing systems of the type described above are described in Blanchard U.S. Pat. No. 4,334,552 and Bolling et al U.S. Pat. No. 5,295,510. In the diverter valves of both patents, polymer enters the diverter valve through a lateral port in a valve manifold and exits the diverter valve through two alternative outlet ports disposed generally at the same outwardly facing side of the manifold once of the disadvantages of such arrangement is that the relatively close adjacent disposition of the two outlet ports necessitates placement of the two filter vessels at the same side of the diverter valve which, in turn, necessitates that the piping connecting the diverter valve ports respectively to the filters must be sufficiently long to permit the filters to be spaced from one another.